1. Field of the Invention
This invention relates to a radiation image read-out apparatus wherein a stimulable phosphor sheet carrying a radiation image stored thereon is scanned with stimulating rays which cause the stimulable phosphor sheet to emit light in proportion to the amount of energy stored thereon during exposure to radiation, and the emitted light is photoelectrically detected in order to read out the radiation image. This invention particularly relates to a radiation image read-out apparatus wherein a stimulable phosphor sheet is held on a concave cylindrical surface and scanned with stimulating rays radiated from a rotating member which rotates around the center axis of the concave cylindrical surface.
2. Description of the Prior Art
When certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object such as a human body in order to store a radiation image of the object thereon, and is then scanned with stimulating rays such as a laser beam which cause it to emit light in proportion to the amount of energy stored during exposure to radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal, and the electric image signal is used to reproduce the radiation image of the object as a visible image on a recording material such as photographic film, a display device such as a cathode ray tube (CRT), or the like.
Stimulable phosphor sheets are used as a means for recording radiation images mainly in the field of medicine, but they may be used in various other fields as well. For example, the applicant proposed in U.S. Pat. No. 4,651,220 and U.S. Pat. No. 4,889,990 novel methods for recording and reproducing an electron microscope image wherein a stimulable phosphor sheet is utilized. Basically, the proposed methods for recording and reproducing an electron microscope image comprises the steps of (i) exposing a stimulable phosphor sheet for storing electron beam energy thereon to an electron beam which has passed through a sample in a vacuum in order to store the electron beam energy on the stimulable phosphor sheet, (ii) thereafter exposing the stimulable phosphor sheet to, for example, stimulating rays to release the stored energy as emitted light, (iii) photoelectrically detecting the emitted light to obtain an image signal, and (iv) using the image signal in order to reproduce an electron beam image of the sample.
It is desirable that a read-out apparatus, which is used for the aforesaid electron microscope image recording and reproducing methods in order to irradiate the stimulating rays to the stimulable phosphor sheet and to detect the light emitted by the stimulable phosphor sheet, be capable of scanning the stimulating rays at a higher density than the density at which the read-out apparatus for the aforesaid systems for recording and reproducing the radiation image of a human body or the like scans stimulating rays. Also, it is desirable that the former read-out apparatus be capable of detecting recorded images at a higher accuracy than the accuracy at which the latter read-out apparatus detects recorded images. In order to satisfy the aforesaid requirements, a novel radiation image read-out apparatus has been proposed in Japanese Unexamined Patent Publication No. 59(1984)-13235. The proposed radiation image read-out apparatus comprises an optical element, such as a dichroic mirror, which reflects or transmits stimulating rays and transmits or reflects light emitted by a stimulable phosphor sheet, and which is positioned between a light source and the stimulable phosphor sheet. The proposed radiation image read-out apparatus also comprises a condensing element (lens) for converging the stimulating rays, which come from the optical element, onto the stimulable phosphor sheet. The stimulable phosphor sheet is formed cylindrically and rotated to carry out scanning in the main scanning direction. In the proposed radiation image read-out apparatus, the light emitted by the stimulable phosphor sheet passes through the condensing lens from the direction reverse to the direction along which the stimulating rays pass through the condensing lens. The light emitted by the stimulable phosphor sheet is collimated by the condensing lens 23, and the collimated light passes through or is reflected by the dichroic mirror. The collimated light is then converged by a converging lens and detected by a photodetector. With the proposed radiation image readout apparatus, an optical element having a short focal length can be used as the aforesaid condensing element, and the stimulating rays can be converged to a small spot diameter. Also, the light emitted by the stimulable phosphor sheet is condensed by the condensing lens and the converging lens, and therefore no shading arises as in cases where a large light guide member is used.
Also, a different type of an image read-out apparatus is disclosed in Japanese Unexamined Patent Publication No. 52(1977)-16123. In the disclosed image read-out apparatus, a recording medium is held cylindrically (on a concave cylindrical surface) and is kept stationary. The recording medium is scanned with a light beam radiated from a rotating member which rotates around the center axis of the concave cylindrical surface. The light beam serving as a reading light is caused to scan with respect to the recording medium, and light reflected from the recording medium is detected in order to read out the image information recorded on the recording medium. Such a scanning system is also applicable when light emitted by a stimulable phosphor sheet is detected in order to read out a radiation image stored on the stimulable phosphor sheet. In such cases, as in the cases of the radiation image read-out apparatus proposed in Japanese Unexamined Patent Publication No. 59(1984)-13235, stimulating rays can be converged to a small spot diameter, and shading can be prevented from occurring.
However, in cases where a stimulable phosphor sheet is held on a concave cylindrical surface and scanned with stimulating rays radiated from a rotating member which rotates around the center axis of the concave cylindrical surface, if the optical axis of a laser beam serving as stimulating rays deviates in parallel from the center axis of the cylindrical surface due to, for example, pointing of a laser beam source, the efficiency with which the light emitted by the stimulable phosphor sheet is detected will fluctuate among the positions at which the stimulable phosphor sheet is scanned with the laser beam in the main scanning direction, and shading will arise. The problem will be described hereinbelow with reference to FIG. 5 which is a partial plan view showing the conventional radiation image read-out apparatus.
With reference to FIG. 5, a stimulable phosphor sheet 70 is held with a recording surface thereof facing a center axis L so that the stimulable phosphor sheet 70 is cylindrical around the center axis L. Both edges of the stimulable phosphor sheet 70 are denoted by 70L and 70R. A deflection mirror 71 rotates around the center axis L. Stimulating rays 72 such as a laser beam advance along the center axis L, and is reflected by the deflection mirror 71 to the stimulable phosphor sheet 70. A condensing lens 73 is combined integrally with the deflection mirror 71, and the stimulating rays 72 are converged by the condensing lens 73 onto the stimulable phosphor sheet 70. When the stimulable phosphor sheet 70 is exposed to the stimulating rays 72, the exposed portion of the sheet emits light 74 in proportion to the amount of energy stored thereon during exposure to radiation. The emitted light 74 is collimated by the condensing lens 73 and advances along the optical path of the stimulating rays 72 in the direction reverse to the direction along which the stimulating rays 72 advance. The emitted light 74 is separated by an optical element 75, which is constituted by a dichroic mirror or the like, from the optical path of the stimulating rays 72. The emitted light 74 is then converged by a detection lens 76, and detected by a photodetector 77. In order to keep the sharpness of the read-out image high, an aperture plate 78 which allows only the emitted light 74, after being converged by the detection lens 76, to pass therethrough is positioned in front of the photodetector 77.
In cases where the optical axis of the stimulating rays 72 coincides exactly with the center axis L, the stimulating rays 72 scan the stimulable phosphor sheet 70 along a straight scanning line H. However, in cases where the optical axis of the stimulating rays 72 deviates in parallel from the center axis L as indicated by the broken line, the stimulating rays 72 scan the stimulable phosphor sheet 70 along a bent scanning line J. Specifically, when the stimulating rays 72 are reflected by the deflection mirror 71 to the upper side in FIG. 5, they advance along the direction indicated by the arrow K. When the stimulating rays 72 are reflected by the deflection mirror 71 to the lower side in FIG. 5, they advance along the direction indicated by the arrow M.
In order to collimate the emitted light 74, the condensing lens 73 is spaced by a distance equal to the focal length f (with respect to the emitted light 74) from the stimulable phosphor sheet 70. In the strict sense, because the wavelengths of the stimulating rays 72 are different from the range of wavelengths of the emitted light 74, the stimulating rays 72 are converged by the condensing lens 73 to a position slightly deviated from the surface of the stimulable phosphor sheet 70. Even in such cases, if the optical axis of the stimulating rays 72 always coincides with the center axis L, the optical axis of the emitted light 74 after being converged by the detection lens 76 will not deviate from the center of the aperture of the aperture plate 78. However, when the position at which the stimulating rays 72 are converged deviates from the surface of the stimulable phosphor sheet 70 and the optical axis of the stimulating rays 72 deviates from the axis of the condensing lens 73, the optical axis of the emitted light 74 after being converged by the detection lens 76 deviates as shown in FIG. 6 from the center of the aperture of the aperture plate 78. The extent to which the optical axis of the emitted light 74 after being converged by the detection lens 76 deviates from the center of the aperture of the aperture plate 78 fluctuates depending on the extent to which the optical axis of the stimulating rays 72 deviates from the axis of the condensing lens 73. Therefore, the amount of the emitted light 74 which passes through the aperture of the aperture plate 78 and is detected by the photodetector 77 fluctuates as the position changes at which the stimulable phosphor sheet 70 is scanned with the stimulating rays 72 in the main scanning direction.